The invention relates to a process for etching glass or SiO2 with metal present wherein the metal is not etched during the etching of the glass or SiO2.
It is customary in the art to etch glass (amorphous SiO2 films) with an aqueous solution of an acid, for example hydrofluoric acid, buffered with, e.g., ammonium fluoride. In the semiconductor industry, it is commonly necessary to etch semiconductor materials like glass formed on a metal substrate, such as aluminum, or to etch glass having metal conducting lines on a silicon wafer or die. The acid during the etching process attacks and corrodes the metal.
It has been reported that the addition of an expensive diol alcohol to a buffered acid etching solution inhibits its attack upon the metal. Moreover, it has been reported that a suitable amount of diol alcohol, e.g., ethylene glycol, to obtain this result is somewhere on the order of fifty percent of the etching solution. For example, it has been stated in U.S. Pat. No. 4,040,897 that a typical formulation using a diol alcohol to inhibit attack upon the metal is comprised as follows: five parts of a buffered oxide etch (xe2x80x9cBOExe2x80x9d) comprising thirty-four parts by weight of a forty percent aqueous ammonium fluoride solution, five parts of a forty-nine percent aqueous hydrofluoric acid solution, five parts of ethylene glycol and three parts water. In the same patent, it is reported that the diol alcohol containing etchant solution is nearly saturated and that it has a high viscosity which renders it difficult to rinse off and/or to etch small openings in a photoresist. It is also reported that the diol alcohol mixture tends to corrode the metal intermittently.
U.S. Pat. No. 4,040,897 titled xe2x80x9cEtchants for Glass Films on Metal Substratesxe2x80x9d discloses a method of etching a glass layer on a metal substrate comprising contacting the glass layer with a dilute aqueous etch solution for a sufficient time to etch through the glass layer. The etch solution disclosed in U.S. Pat. No. 4,040,897 is comprised of a buffered hydrofluoric acid, sodium chloride, and a fluorocarbon surfactant. However, it has been found that using the process disclosed in U.S. Pat. No. 4,040,897 does not completely protect the metal from attack and thus corrosion. Indeed, it has been found that upon exposure to the etchant solution and before the surfactant can coat the metal, the metal is attacked by the acid etchant and corroded to some extent. In addition, it has been found that the above-discussed processes do not protect the metal substrate from galvanic action which is frequently encountered in wet etching processes with semiconductor materials.
The invention is directed to an etching process for semiconductor materials which protects metals from corrosion comprising: (1) pretreating the material to be etched (to protect the metal), and (2) exposing said material to a mixture of a buffered oxide etch and a salt. The salt is added in a sufficient amount so that the mixture maintains a natural saturation equilibrium during the etching process or any electrolysis effects are neutralized. Preferably, the material is pretreated with a surfactant. Alternatively, the process of the present invention may include pretreating only the metal with a surfactant. In addition, the exposing step may include a surfactant.
The invention is also directed to an etchant for semiconductor materials comprising: (1) from about 0.03 to about 0.06 percent by weight surfactant, (2) from about 15 to about 30 percent by weight buffered oxide etch, and (3) a salt in sufficient quantity to neutralize electrolysis effects.
The invention is further directed to a semiconductor material etched by the process of the present invention. The process comprises: (1) pretreating the material with a fluorocarbon surfactant, and (2) exposing said pretreated material to an etchant bath comprising from about 2 to about 4 percent by weight salt, from about 16 to about 28 percent by weight buffered oxide etch, and (3) agitating the bath. The etchant bath is preferably comprised of sodium chloride in an amount such that the bath maintains a natural saturation equilibrium. Optionally, if desired, the etchant bath may include a surfactant.